In recent years, lubricating oils for internal combustion engines are strongly required to improve fuel economy characteristics, and as one means, reduction of friction loss due to lowering viscosity of a lubricating oil is cited. However, merely lowering the viscosity causes problems such as leakage and burning, so it is effective to add a viscosity index improver having the effect of keeping the viscosity low at low temperature while ensuring high viscosity at high temperature.
As the viscosity index improver, a viscosity index improver containing a polymer is known, and there are various kinds of them. Among them, a viscosity index improver comprising an alkyl (meth)acrylate polymer shows a high viscosity index improving effect. Meanwhile, the viscosity index improver comprising an alkyl (meth)acrylate polymer has a problem that shear stability is poor so that fuel saving characteristics are deteriorated for a long-term use (that is a poor long-life property).
As a means for improving the shear stability, for example, decrease of molecular weight of the polymer contained in the viscosity index improver is cited. In general, the lower the molecular weight, the less susceptible to shear and the smaller the degree of the molecular weight decreasing; and therefore, it is possible to suppress the viscosity lowering after shearing by using a low molecular weight viscosity index improver (refer to Patent Literature 1 and Non-Patent Literature 1). In addition, it has been reported that attempts have been made to improve the shear stability with a polymer having a star structure of which divinylbenzene is used as a core part (refer to Patent Literature 2).